If your motorhome’s Combi heater is drawing in air from within the storage area beneath the bed, then adding vents into that storage area will not allow escape of the heat produced by the heater itself and the warm-air trunking, as the heater will then draw air into the storage area through the vents.
However, if the Combi is drawing in most (or all!) of its air from within the under-bed storage area, rather than from the motorhome’s living-area outside the under-bed storage area, then adding vents (preferably positioned near the Combi’s air-intake fan) will - as suggested - allow the heater to perform its design-objective of recirculating warmed air around the motorhome’s living area. So, if the air in the under-bed storage area is getting very hot due to the heater primarlly being unable to obtain cooler air from outside the storage area, instalingl vents into the storage area should increase the heating system’s efficiency and might help to reduce heat build-up beneath the bed.
Truma’s advice on fitting a Combi heater is often ignored completely by motorhome converters, with the result that the standard of a Combi-based heating-system's installation generally varies from just-about-adequate to atrocious. Correcting all the design faults in a poorly-executed installation is usually impracticable, but it should be possible to make useful improvements - and ensuring that the Combi heater can ‘breathe’ efficiently is the first thing to do.
When you are using the Combi just to heat water in the summer, it will still radiate a good deal of heat into the under-bed storage area, and you are unlikely to want to extract that heat into your Tracker’s living-area - so your extractor-fan idea would not be applicable to warm weather usage. You could try adding a fan (I can’t see it doing any harm) but I suggest you address any obvious shortcomings in your motorhome’s heating installation first.